The invention relates to driving a display such as a plasma display panel.
An (AC) plasma display panel (PDP) and a digital (micro-)mirror device (DMD) are bi-level displays with a memory function, i.e., pixels (picture elements) can only be turned on or off. In conventional PDPs, three phases can be distinguished; an erase sequence, an addressing sequence and a sustain sequence. In the first sequence, the memories of all pixels are cleared. To switch a pixel on, the second addressing phase is necessary. In such a phase, the pixels are addressed on a line at a time basis. The pixels that should turn on are conditioned in such a way, that they each turn on when a voltage is put across its electrodes. The conditioning is done for all pixels in a display that should be switched on. After the addressing phase, a third phase, the sustain phase, is required in which the luminance is generated. All pixels that were addressed, turn on as long as the sustain phase lasts. The sustain period is common for all pixels of a display, thus, during this sustain period, all pixels on the screen that were addressed are switched on simultaneously.
The field period is divided into several sub-fields each consisting of a sequence of erase, address and sustain. The grey-scale contribution of each sub-field is determined by varying the duration of the sustain phase, i.e., how long the pixels are switched on. The duration of the sustain phase is further denoted as the weight of a sub-field. The higher the weight of a sub-field, the higher the luminance of a pixel that is switched on during the sustain phase. The grey-scale itself is now generated in such a way that the luminance value is divided into several sub-fields in which the sub-fields have various weights, i.e., the duration of the sustain phase is proportional to a weight factor, thus, also, the luminance output is proportional to the same weight factor. The sub-fields can be started in two fashions; they can be equally divided over a field period, or they can start when the previous one is finished. The latter situation is shown in FIG. 1. In FIG. 1, a field period including six sub-fields SF1-SF6 is shown for a conventional PDP. Each sub-field SFi includes an erase period EP, an addressing period AP, and a sustain period SP. The length of the sustain period SP of a sub-field determines its impact on the output luminance. By combining the sub-fields (i.e., switching the sub-fields on or off), a grey-scale can be made.
FIGS. 2A-2D show the artifacts resulting from motion at a speed of 2 pixels per field period. FIG. 2D shows a Time vs. Position diagram in which the six sub-fields together forming a first field T0 are shown on the vertical axis, and position P is shown on the horizontal axis. Increasing luminance values L are set out horizontally; these luminance values are built up in a digital manner by means of the various sub-fields having binary weights. FIG. 2C shows where the various sub-field informations are perceived as a result of the motion at 2 pixels per field period. FIG. 2B shows the luminance contributions of the individual sub-fields, in which the sub-field T5sf with the weight 25=32 is shown as the largest pillar, and the sub-field T0sf with the weight 20=1 is shown as the smallest pillar. FIG. 2A shows the resulting luminance on the retina, as well as a line R indicating the intended ramp. The difference between the intended ramp and the actually perceived luminance on the retina is a problem to be solved. It can be seen from FIG. 2A that the observed luminance can differ a lot from the actual still image data. This method calculates the precise position of the sub-fields and weights of the pixels under the assumption that the eye is tracking the motion according to the motion vectors. FIG. 2D shows a part of the black and white luminance ramp. In this time-position diagram, the motion vectors are drawn with a speed of 2 pixels per field period. The projections of the separate sub-fields are drawn on a diagram in which the luminance is drawn as a function of the position on the retina when the eye is perfectly tracking the motion with a speed of 2 pixels per field period. All luminances generated by the sub-fields that are received at the same positions on the retina are integrated resulting in a diagram in which the total luminance received by the retina has been drawn as a function of the position on the retina (this is shown in FIG. 2A). What can be seen is that the pattern on the retina still does not resemble the still image luminance ramp. There is still a bright vertical bar visible. This is the cause of contouring, there is only a slight change in luminance between two pixels which result in a perceptive bright or dark impression. What also can be seen is that there are gaps visible between the MSB sub-fields. These gaps are only visible from a close distance and are caused by the black matrix in between the pixels. From a greater distance these gaps are not visible any more which can also be said when the bright vertical line gets too small. What can be seen from this figure is, that it looks like the luminance contributions of the sub-fields are not projected on the same positions as the most significant sub-field weight. It is as if some sub-fields take positions in between the pixels which is, in practice, not possible due to the discrete character of the display. This phenomenon is also explained in [Mikoshiba2]. This is all due to the low-pass behavior of the eyes, which give the suggestion that all sub-fields are generated at the same time which is not true.
As known from the prior art, motion-compensation can help reducing the motion artifacts. In the Time vs. Position diagram of FIG. 3, compensation of a grey level of 20 is shown for two successive fields T0 and T1. OL indicates the observed luminance, OP indicates the original positions. Without motion and thus without motion-tracking by the eye, the values 4 and 16 are on top of each other and thus added: the correct luminance value of 20 is observed. When a vertical line with this grey level moves over the screen with a speed of 6 pixels per field period, a motion artifact is seen of two vertical lines with a luminance level of 16 and 4. So, with motion and thus with motion-tracking by the eye but without motion-compensation, two separate lines are observed: a 16-line and a 4-line. This problem could be solved by shifting the sub-field with a weight of 4 to the right to the position where this sub-field crosses the motion vector (the time at which this sub-field is generated). So, with motion-compensation, the 4-values are shifted to the 16-line, so that the motion-tracking eye again perceives the correct value of 20. When the luminance variations are determined by amplitude modulation as on a CRT, the luminance is generated on one position on the retina, and when this movement is being tracked, the same luminance is again generated on the same position on the retina. Since, on a plasma display, the grey-scale modulation is done on a sub-field basis, and this object needs to have the same luminance during tracking, it is required to generate these separate sub-fields on the projected motion vector. When doing this, it can be seen from FIG. 3 that no longer two vertical lines are observed on the motion vectors, but only one with a luminance of 20.
It can also be seen that to be able to do this, it is required to assign two vertical lines to two columns of pixels, i.e., one column is assigned the value 16 and the other gets the value 4. When inspecting one field of this image, two vertical lines are seen, but when the whole moving sequence is observed (and this sequence is tracked by our eyes), only one vertical line is seen. Thus, to compensate for the error introduced by the motion and the tracking of the eyes, a luminance of 20 must be shown as projected on the motion vector. Thus, by shifting the luminance level of 4 to the right to a position on the motion vector, the right luminance level of the vertical line is obtained, when this pattern has a speed of 6 pixels per field period to the right.
The same method can be used for a luminance ramp. To compensate for this pattern; the luminances that are required are the luminance levels shown on the motion vectors, i.e., the luminances of the pixels that are shown are the luminances of the compensation pattern. This is shown in FIG. 4, in which OL indicates the obtained luminance when tracking, as a result of putting not the desired ramp itself, but the compensation pattern CP on the display. Thus, the luminances of the pixels that are visible, are the luminances projected on the motion vectors when the eyes are tracking the motion of 6 pixels per field period. What can be seen from this figure is that, when inspecting one field of this sequence at one position, a dark luminance level of 2 is shown, as, in this case, not the tracked motion, but the luminance of the compensation pattern CP is observed.
So, motion-compensation could work, but there is a problem in doing this for an arbitrary speed, as illustrated in FIGS. 5A-5D for a luminance change from 31 to 32 which is moving to the left with a speed of 3 pixels per field period. On the boundary of this luminance change an artifact is still clearly visible. This can be explained as follows. When the plasma panel has 6 sub-fields equally divided over one field time, and there is a speed of 6 pixels per field period, this results in a speed of 1 pixel every sub-field. Thus, motion-compensation works almost perfectly since the sub-field weights can be shifted to subsequent neighbor pixels. So, the sub-fields are exactly located on the motion vector and the grid of the matrix display. With an arbitrary speed, this no longer holds, and it is necessary to map the sub-fields to pixels that are not exactly located on the motion vector, so that other some artifacts remain.
It is, inter alia, an object of the invention to provide an improved method of driving a display which results in less visible artifacts. To this end, a first aspect of the invention provides a method of driving a display. Further aspects of the invention provide a display driving device using the method and a display apparatus incorporating the display driving device.
In a method of driving a display in accordance with a primary aspect of the present invention, field information from a field of an image signal is distributed over a plurality of sub-fields, and a start time for each sub-field is generated in dependence upon motion.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.